Well I think if you need more shade in the cockpit you should extend or modify your bimini, but don't alter your boom. Remember that the boom is only meant to support the foot of the mainsail. It would be difficult to add to it. If it were altered to lengthen it, the boom would likely have to be replaced. But that is not necessary. Focus on the shade that a good bimini provides, which is invaluable. The only issue you will have with extending your bimini with a stainless steel type tubing is working around your backstay. Most larger biminis have a leather lined hole cut out for the backstay to run throught the bimini. It is tensioned & secured on the aft side with a heavy duty zipper. Biminis are expensive. Custom canvas is outrageously expensive. I'm a DIY kinda sailor, so we bought a heavy duty sewing machine to make our own canvas covers, etc. If U can't afford to make one, then scope out old dock yards for boats that look to be growing roots & abandoned. Most of the time the marina / yard owns them by default for non payment of rent. Many times they'll let you buy bits & pieces off of their golden oldies for next to nothing. There may be good canvas laying around inside the cabin of an old hulk that might still be good. Just don't do what a fella in my marina does & collect old run down boats! Good luck.
ps. You can also buy used port holes from marine chandleries who have bought suff from parted out boats. Used ports should cost no more then $50 bucks a piece, unless they're bronze.