At gas stations people play a surcharge for paying their bill with credit cards. Law could provide that pre paid cards cannot be used for sending (as) remittances unless the fee were paid in advance and paid to the feds.
Postal money orders have the name and address written on them and the PO could charge the fee and remit to the feds.
If the remittance fee were something like between 2% and 7% it would probably not cause a mass exodus. The estimated remittances are $26billion. Four percent would provide $1billion a year. With the the legal work needed to build the wall it will take a while. Legal work like eminent domain cases, negotiating for water rights for people along border rivers, payment for land rental, dealing with local, state, and national park issues, etc. will slow down the actual building time.
I like the idea of a tariff it is more in your face Mexico. I don’t care if there is a one time 20% raise in imported goods form Mexico. Worth it IMO. I buy a bottle of tequila once a year. If a tariff shuts all those factories dumping product into the USA then hurray for that!