Paul; Check the electrical departments at Home Depot and Lowes for inexpensive plastic boxes made by Carlton. These are fully weatherproof boxes without any knockouts that are excellent for weatherproofing preamplifiers. You will probably need some type N bulkheads that have a long neck so that you can pass through the plastic plus whatever aluminum gauge ground plane you might want to bond your preamps to. I just got through rebuilding a wideband amplifier 25 MHz -1 GHz for my receiver, and I have a second one built at least 12 years ago for 240-270 MHz that although the box is pretty weathered, the electronics inside including a bandpass graph and notes on paper inside, look like new. Absolutely no water intrusion although outside for years. Get a bigger deeper box than you think you need. I used 6x6x5 inch boxes.
For this one I used a Polyphazer "pickoff" to extract 15V DC from the coax to run a gasfet preamp and a WJ 28 dB gain T08 amplifier.