In search of the featherweight tribander

I've not given up on the idea of a very small, very light-weight tribander (10m/15m/20m) antenna for my 100 Pound DXpeditions. I had purchased a C3SS from Force-12 for this purpose, and it was a very good performer, but it was just too large (and a bit complicated) for the kind of duty I had intended. I'm still looking. The constraints for my search are these:

• Under 30 pounds - Actually, under 25 pounds would be better, and something under 20 pounds would be excellent. Is this possible?
• Easily assembled - The C3SS was a fantastic performer, but I wouldn't want to be putting that together on a windy beach on some far-away place. Ideally, I like something that slips together in some obvious way in about 30 minutes or less.
• Breaks down small - How small? No piece can be longer than 48 inches in length, the size of my hard-sided golf bag. Even a little shorter would be better (perhaps 40 inches?)
• Some gain - I'm going to be realistic here. Gain is achieved by boom length, among other things, and my constraints are going to mean a very short boom. Still, it would be nice to have some significant gain on 15m and 10m, even if the gain on 20m is small.
• Driven with 50 ohm feed - I just want to hook my coax directly to the antenna. Whatever matching system is there (hairpin, choke, etc.) should be small and no-fuss.
• Good front-to-back ratio - Even if I'm not getting lots of gain, it would be nice if the pattern of the antenna rejected signals I wanted to ignore anyway.
• Cheap - I'm not made of money. Gee wizz, it is only aluminum! How much can such a thing cost?!

There are some things I'm willing to give up to help make this happen. Here are a couple of them:

• Wind survivability - I don't care. If the wind blows hard, I will take it down. I'm using this for travel. It doesn't need to be strong enough to live on a tower 12 months a year. It could even be a little flimsy, if it meant saving weight and cost.
• Power rating - This is a big one. I don't need it to handle full legal limit power. I don't even need it to handle 600 watts of power. It could be rated at 120 watts SSB, 80 watts CW, and 50 watts continuous (for RTTY or PSK-31) and it would be more than sufficient. There aren't many antennas out there for operators who run QRP or low power (100 watts). It seems to me this would be a big savings in the traps, for example.

At this point I believe I have two choices: buy some commercial antenna off-the-shelf and cut it down for travel, or design one from scratch (even if I use some off-the-shelf components). If I had a workshop and a place to work and test such creations, I would be tempted to make my own. Unfortunately, I don't have such facilities in my condominium. So, I think I'll be buying something.
I'm leaning towards the Hy-Gain TH-3JRS which weighs 21 pounds according to the web site. It handles 600 watts and has a 12 foot boom. I would have to hack up the 6 long element tubes (6 foot long each) and the two boom tubes (also 6 foot long). Adding the additional material needed to then join these pieces together would add additional weight, but I don't see another way to do it.
The antenna is about $360 and there would be additional expenses trying to figure out how to make it smaller. That's a pricey experiment, but I'm considering it. I'll be mulling this over in the next few weeks.