Hi Thundirbirds,
You could increase the amount of water by moving the forward bulkhead forward during construction. The Seamaster kits have long since been out of production so getting an unbuilt kit may be difficult. As for "dropability", the door I made was as big as I could fit on the step. You could change the door location (perhaps to the bottom) but this would require a bit more engineering and enginuity.
I have had a lot of fun with mine but it has not flown in several years. Sits on my shelf in the work shop actually! I had hoped to get to more float fly's but it just hasn't worked out. [

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Dave