How to isolate bookshelf speakers on a bookshelf

Yes, I know that bookshelf speakers should never be placed in a bookshelf. My situation is a bit limiting. I temporarily need to place my Triangle Titus 202 speakers on the top of a 30-inch high wood credenza/bookshelf, which runs the entire length of the wall.

So they are surrounded by air on the top and sides, and are about 8 inches from the wall.

What is the best way to improve the sound? Should I isolate the speakers from the base using bits of Blu-Tack, or use isolation cones?

Since this set-up will only last a year or so, I don't want to invest too much $$$, but do need to improve the sound.

Also, what are the acoustic reasons why placing the speakers on the top of a shelf degrades the sound versus using quality speaker stands? Is it that there is room for the sound waves to move more freely, or that the speaker cabinet needs to be isolate from resonant objects? Or both?

On top of a cabinet is much better then in a bookcase.
Blue-Tack will couple the speaker to the cabinet. Footers will have a different effect. Try both.
You can experiment with Vibrapods and Vibrapoints for relatively little money, and later use them under other components.
I know it's a pain in the ass when you have limited placement options.I have dealt with that before.With the 202's being front ported you can use them relatively close to the wall.I would not put them in a "cubby hole" though.Be aware, if you use vibrapods,they will stain whatever the rubber is touching.Try some of the felt pads that are used to protect wood floors.Good luck.
Go to a place that sells foam rubber mattresses and get some pieces of scrap foam. They should be cheap/free. Put them under the speakers and see if it helps. If it does you can use them or pursue a higher tech solution along same lines. Have you considered suspending them from the ceiling using some of the nets used for plants? I once heard LS3/5As set up in this manner and it worked quite well.
I had a setup at my old house with Von Schweickert VR1s in my built in bookshelves. I used a quasi blu-tack product I got at my hardware store (it is blue and can't remember the name but you find it where they have the picture frame hooks)and it worked great. It comes in flat strips. I just cut 4 squares for each speaker and stuck them on the speaker, then positioned the speaker where I wanted. It tightened and warmed the bass at the same time. No issues of marking speaker or shelf, or paint peelup.
I think it's because the sound reflects off the surface right in front of the speaker, so they would sound better if you placed them at the front edge of the surface.
I've used Black Diamond Racing Cones and liked it. Also examine the stuff at Herbie's Audio Lab. All these are not very expensive and sell used for 60% of list in a heartbeat so your overall out of pocket will be very low.

With monitors on bookshelves you lose imaging but the tonality can sound fantastic if you correct for bass issues.

I'm assuming that your speakers are FRONT-PORTED.

If they are rear ported you will need to do a lot of experimentation with angles and distance from wall, and, may just end up with a compromise.

Usher s520 are front ported and beautiful in black and sound very lovely, AND seem to hold their used value, in case you need to make a purchase to get you through this year.

Moon Gel is the way to go. This product will isolate better than anything on the market, very inexpensive too. It can be found at the pro music stores, 123 Music or Musicians Friend, in the section where drummer acessories are sold. Go on line to check it out. Blue in color, each pad is about one inch square. Finally, as noted by others, since the 202's are front ported it may work for you.
Another vote for Herbie's Audio Lab products. His Grungebuster Dots work very well under my monitors.
Any visco-elastic material lowers the natural frequency and dissipates vibrations into heat. All these diamond racing cones, vibrapoints, blue tacks, neoprene, etc., work on the same principle.
Because of space limitations, I have had my bookshelf speakers mounted on a piece of furniture for some time. I have tried both coupling and decoupling approaches. The best solution for me was a combination of Outriggers and Black Diamond Racing cones/pits. The results are quite good, given the limitations. You can see pictures of my solution in my virtual system. Good luck with it.
Auralex MoPad Monitor Isolation Pads