I took a look just to see what I could find and the Tent /Canopy Sandbags look like a good start and I can get them for under $20
This is pretty much the concept of how I may try to weigh the base. This would still give me room to weigh the legs with similar bags maybe smaller or the same size. It might even be durable enough to do low leg kicks even though they aren't made for that. If I design the base correctly I should be able to create a multi-use training equipment.
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From this picture, the center of gravity is too high. It might work holding the tent down, but if you punch and kick high, it's going to tumble over. You need to have something that has very low center of gravity. That's the reason those slam man has the water/sand at the bottom. Even at that, water is no where heavy enough. Even I can punch and knock it over with water with one punch. I bet it won't hold up at all for you. I had two of the water base ones and I gave them all away. I don't want to use sand because it's too heavy to move it. I have heavy bags hanging already, no point of trying too hard with those.
It is not easy, you said you have banana trees, see whether it is strong enough to hang a heavy bag. A big tree is your best bet......But then, what if it rains? Those vinyl cover bags won't stand up for $hit, you have to have either canvas or leather to hold up. You'll break those vinyl in weeks. But canvas or leather is not water proof.
You need at least a 70lbs bag, those 40lbs are not going to do it for you. It's too heavy to carry it back to the house every time it rains. A lot of those water base stuffs seems to be water proof. The slam man definitely water proof(without the electronics), the other one I have is Chuck Norris one I bought long time ago, it's vinyl though.
Punching and kicking is very harsh.
What is the reason you don't want to use cement base into the ground? Only cement with a big pole can do it, you still better dig deep, don't think they cannot be topple over. I am not even comfortable using a 6"X6" wood pole. Remember my video of punching the pole, it's about 6X6, it's anchor on both top and bottom. I don't even punch hard, but the whole thing vibrates when I punch. If it is anchor only at the bottom, you are going to break it at the base sooner or later if you starting to do high side kicks.
Another note: If you are tying the sand bag on the pole like in the picture, it won't move when you punch and kick. It might be very damaging to your joints. Hanging bags move upon impact. Punching is ok as you are not suppose to move the bag, but for side kick, if it doesn't move, it's going to hurt your knee, hip and back. It's very different kicking the 100lbs bag vs 70lbs even they are both free hanging. Just because the 100lbs move less, I can feel the impact on my knee, foot and all. Watch out if you punch and kick an unmovable object.