You could do worse than to look for a different, local speaker - either 8" or bigger in in a cabinet - but of course that's what you are doing, and you're finding refrigerators. Heck, a car audio subwooferish thing might work, and might be cheap at the automobile junkyard.
What I can tell you, from my own research into my inexpensive and very old-tech combo is that the route to making the next one survive will almost certainly be to crank your bass EQ way down, or (if you have it) your high-pass filter way up. I was somewhat less than enthralled to find that my "200W 1x15" speaker coupled to a 130W amp will die with anywhere from 25-40 watts (depending what box it's in - I was looking at the "build a better, lighter box for it" option) at 65-80 Hz. Fortunately, I haven't found that out by blowing it.
As it happens, it has a setup to split for bi-amping in the preamp section, and if I take the high part of that and set the crossover at about 125 hz, I can wail the full 130 watts the amp makes into the speaker without issue - if I had a 200W (yes, I know YOU have a 300W amp, but the speaker I have here is only 200W) amp, it would need to be set at a bit higher frequency to be safe. The difference in sound is actually not as bad as you would think - it's worth trying if you have the opportunity. It loses some room-vibrating ability, but does not sound nearly as weak as you might think it would, and won't blow the speaker (or so says the simulation software...)
If you can do something like that, you might even get away with using an old high-fi speaker for a while (yes, yes, terrible idea for the purists, but I get that your speaker is blown and you are looking for _something_ that will work NOW, so see what you can find cheap and used even if it doesn't claim to be "a bass speaker"), and you could eventually add a sub if you felt the need to fill in the bottom. A PA speaker would be a more "musician approved" option in a similar vein.
In general, to get more use out of your amp, you'll want more speakers, period. If you use smallish ones, you might not need two roadies, though you will need two or three trips, or a cart to haul them. If you get 4 the same, you can wire them up to be the same impedance as one.