You'd be far far better off waiting until you can get them to a lab. Given the sensitivity of the technique and the unknown abundances and likelihood of getting samples, giving them the best extraction you can is the way to go. I realize this isn't always feasible and there are reasons not to do an extraction in the lab, but it is generally better. Fixing in ethanol on a filter would be fine to preserve until you can get it to a lab.
It would depend on the scale of preparation you want to do, exactly what you want to get out of it and the abundance of the bacteria in your samples. There are a number of para-magnetic bead based DNA (AKA SPRI beads) and RNA extraction products on the market. These are highly sensitive and quite quick to perform, but you do need the magnets and beads, both of which are quite expensive.
An example of this used for environmental bacteria is the following article. They did their extraction in the lab and used a centrifuge to concentrate samples, but it could easily be applied to the field:
Byrne RL, Cocker D, Alyayyoussi G, Mphasa M, Charles M, Mandula T, Williams CT, Rigby J, Hearn J, Feasey N, Adams ER, Edwards T. A novel, magnetic bead-based extraction method for the isolation of antimicrobial resistance genes with a case study in river water in Malawi. J Appl Microbiol. 2022 Nov;133(5):3191-3200. doi: 10.1111/jam.15755. Epub 2022 Aug 23. PMID: 35946113; PMCID: PMC9804433.