You may find "Firestone" printed on that bag, it's original...
For one season I held off replacing mine by slipping a cover made from flex PVC (used/failed water toys) glued together with plumbers PVC cement. If you try this method, include some "No-Odor" product both in the bag and between the bag and sleeve. This worked well enough for one more season but I new it was a stop gap method.
I purchased and installed:
IMTRA Marine Products
You might try calling the West Marine in Toledo OH, they had two "old stock" items and may still have one. If you call, ask for the manager, they know best what they have.
I chose to install a replacement bag for it's a simple installation, no chance of a "vent plug — tank collapse upon pump out" failure. It took longer to shop for than to install this item.
Take this opportunity to install new non odor permeable hose her is a guide to get started . YouTube - Shields Sanitation Hose Product Overview
Here is a link to a small family company. These are nice folks and boaters too... Dock Boxes, Marine RV Holding & Water Tanks, Shower Sump Tanks, Buoys
They may accept your cardboard mockup and pick out the best match from their extensive stock. If you are sure where you want them, they should be able to install the holes.
Also, all I-28 owners should consider installing a vacuum breaker in the tank vent line. Our '79 I-28 did not have one and on Port tack we filled the bag (scary — a quivering, bulging thirty year old bag of "never get the smell out, dear god I hope this holds till we get to the pump out...) Consider what would happened if the joker valve failed under that condition! :-0
I didn't really want to drill through the cabinetry so I compromised the install of the loop. I attached it the the underside of the deck, as far inboard as possible. When heeling with the rail in the water, the loop is now still above the water (most of the time) and being vented, it won't "siphon fill" the bag.
Very Best Regards,