How do I customize the NTP poll rate?

My experience is with ntp rather than openntpd.

You should know that the NTP protocol usually starts with sending packets every minute (64 seconds), but usually settles in by sending and receiving one packet every 17 minutes for each server you have in your /etc/ntp.conf file. Network outages or an unstable clock in your device may increase this, however. Each packet has a payload of about 68 bytes, so that's probably a little more than 100 bytes, each way, per poll.

The rdate and ntpdate at network up time are good ideas for setting the time. And if that is all you need should work well for you.

If you need to keep the time in sync over an extended time, however, the ntp protocol was designed to do that. The clock in your device will run a little fast or slow depending on how warm or cool it gets, even though it probably has a crystal controlled clock -- and ongoing timekeeping is meant to adjust for that.

Here are some suggestions if you need ongoing timekeeping and wish to reduce bandwidth used. They refer to items in ntp.conf.

Don't use iburst. (But it will take more time to set the time initially).

Increase minpoll upwards from 6 (2^6 seconds is 64 seconds). This reduces traffic when the protocol starts up after the network comes up.

Increase maxpoll upwards from 10 (2^10 seconds is 1024 seconds or about every 17 minutes. You can go as far as 17 to poll about every 36 hours. I haven't tried this myself.

Use only a minimum number of servers, if you had one that can be reliably reached from your embedded devices, that would have been great. The more servers the more reliable the timekeeping can be, but traffic scales up, so it is a trade off.

Some of these answers could win the obfuscated ntp configuration contest.

Use the ntp reference implementation and use iburst to set the time quickly upon booting. After that the you can use the minpoll directive to limit how often ntpd queries remote time servers. This will allow you to take advantage of ntpd's clock disciplining and keep the bandwidth down.

minpoll minpoll
maxpoll maxpoll
    These options specify the minimum and maximum poll intervals for NTP 
    messages, in seconds as a power of two. The maximum poll interval 
    defaults to 10 (1024 s), but can be increased by the maxpoll option to 
    an upper limit of 17 (36 h). The minimum poll interval defaults to 6 (64 s), 
    but can be decreased by the minpoll option to a lower limit of 3 (8 s).

Chrony is a good alternative to the ntp reference implementation. Openntpd is not.

You could do some hackery involving running ntpdate when the cell interface comes up. But if you can, installing chrony would be a better alternative, as it was created with intermittent network access in mind.