There is no specific 'format' required.
On a real PDP-11, what happens at bootup depends on what kind of boot PROM you have installed. In general, for disk-type devices the convention is to load the first block then jump to the code in that block. If you don't have a boot PROM for the device you want to boot from, you may have to toggle in a bootloader yourself (if your PDP-11 doesn't have console switches you can usually "toggle in" code using the console terminal instead).
Here is a web page containing sample bootloader code for various devices (including some non-disk devices):
So effectively the only requirement to be able to boot is, you must work within whatever limits your boot PROM imposes. Typically this means you're limited to 256 words (512 bytes), and sometimes the first word must have a specific value for the boot block to be considered valid.
Beyond the first block (the "boot block"), there are usually no requirements on disk format. Normally the format of the disk will be determined by the operating system you are booting.
If you want to, you can simply have your program occupy the blocks that follow consecutively after the boot block, and put a small program into the boot block that just loads those additional blocks, then jumps to the first one.