Deckman called it: On your existing 2x4 top rail - using a countersink bit, predrill a hole 1" below the top edge at an upward angle every 18" or so. Install your cap rail by running (2) 3" screws per end into the adjacent post (Run screws up from bottom edge of 2x4 cap rail into post to hide those screws). Better predrill those holes too. Run 2" screws up through the angled pre-drilled holes to secure the entire length of the cap rail. No visible fasteners from on the deck (you'll be able to see the holes in the top rail when you're out on your lawn). Another good thing about NOT top screwing is less chance for rot (no topside penetrations means less spots for water to enter) Let me know if you don't understand this... Mac

I've used this polycarbonate roofing panel in a railing system before: https://www.homedepot.com/p/Suntuf-26-in-x-8-ft-Polycarbonate-Roofing-Panel-in-Clear-101697/100021329 It is hard to make out what's on the other side. You could also look at a double wall panel or frosted glass.

I can vouch for Stubby and Black Locust Company being awesome to collaborate with! Unlike contractors who take months to get back to you, Stubby called me back within minutes! In talking to him today (early January), he said the lead time is 2-3 weeks during the slower months (winter) and up to 8 weeks during peak months (summer). He noted that you can use black locust for anything except structural support so decking boards, stair risers, and railings. Good luck with your project @Tammy.

I understand that OP needs it to be cheap until closing because this is a lender requirement (not unusual) and if it is fixed before closing, it becomes a fixture and stays with the property even if the buyer doesn't close. CONSIDER: A solution to this would be a rehab type loan rather than the typical purchase loan. Benefit to the buyer is they don't need to resort to expensive hard money type loan as mentioned up thread. Also it gives the buyer the TIME to figure out a permanent solution. The buyer can close right away on a rehab loan. The disadvantage to a rehab loan is that the interest rate is slightly higher than your typical purchase mortgage. Something to explore.