I've made a whack of mousse can after mufflers which really quieten things down. I see on E-Bay there's always these silicone tuned pipe looking after mufflers for sale. They appear to be available in 3 different sizes, the smallest appearing to be ideal for small .15 to .25 size engines... from their looks I'd say they'd do a surprisingly good job of quieting things down. If you try 'em out report back how they work. I know the mousse can idea works very well - I've even built mufflers using smaller aluminum shaving cans, obviously intended for travellers. Check out the (hopefully) attached image to see how I do it... An aluminum plug is inserted half way dow the tube which forces the exhaust gases to travel through the holes in the centre tube. The gases expand in the can of course and the tube acts as baffling. The tube is epoxied to the can using JB Weld. If you use aluminum tubing and cans, the end result can be quite light, very quiet and have minimal impact on performance. I've used them as after mufflers (attached to the stock muffler) and as the primary muffler on a header. You can find aluminum cans like this in a variety of sizes suitable for a wide range of engines. I dare say I think they work better than anything you can buy.