My thoughts are slightly different from some of the other guys..so take them with a pinch of salt if you wish.
I use my wooden floored 4.3m Sib in any weather..therefore I know how much a pounding the floor gets in a good chop. There is no way I would trust home made repairs to my stringers. If they break at sea in a bad weather day..and even the best can get caught in one of those days.. my boat would then be very unseaworthy..especially if sharp splinters of either wood or broken ali went through the tubes.
For me.. my life is worth the cost of buying the proper replacement part.
However if your use is only calm waters and not far from the shore..its a different kettle of fish. However if you do get caught out.. and a bit further from safety that you hoped..remember ... those who recommend making stringers..will be safe on land