i know this is too late for you, but it may be helpful for others.
I am a US citizen. I flew from Bangkok to Dhaka earlier this month on Thai Airways. Thai did not ask me if I had a Banglsdesh visa.
On arrival at Zia airport, the far left hand line had a sign that said "visa on arrival" and/or "landing permit." I went to that line. The immigration officer told me I had to pay $50 for a landing permit. He directed me to a bank located behind him in the arrivals area. I had to go to the bank, pay $50, get a receipt, and come back to thr front of the line.
I did that. The officer then took my passport, spent a long time copying info, and stamped me in with a 15 day landing permit.
I walked outside, agreed with the most reputable looking taxi tout on 500 taka to go into town (Gulshan) in a crappy taxi, and made it there without incident.
