All the siphons I've seen are overkill for a small tank, as well as expensive. I just use a 3 foot length of 3/8 OD clear tubing stuck through the bottom of a 1in diameter pill bottle. I put the hole near the edge and make it a tight fit. Then you can tape a long plastic stick (piece of coat hanger?) along the side of the bottle and up on the tubing. Old Fish Lady has a picture of a similar setup that I used for reference.
After some experimentation, I found that this combination of tubing size and pill bottle size gives me enough suction to pull debris out of the gravel without endangering the fish, and it doesn't take too long to siphon. I wouldn't use this on a 20gal (it would take forever), but for my 2.5g it's fine. Total coat $1.50 for the tubing.