If you can't get yoru hands in the fuse why dont cut covering out and then re apply it back on?Just put the battery on top of the fuse and balance the model,then mark the position of battery and cut the covering,install the battery inside and either iron covering back on or make a patch.If you do it on the bottom of the fuse it won't be noticable while model is on the ground...and in the air..who cares,nobody will be able to see.
I never built tower hobbies 40 but i owned one.The guy i bought it from had a little door cut in the bottom of the fuselage and he had like a tiny door which was held in place with tiny servo screws.

Try it.