If history is anything to be trusted, it's totally fine to post them on full sites' Community Ads topics... posting them in places where you might find supporters is one of the reasons they exist.
The proposal has since been deleted due to lack of support but the Film making proposal was advertised on M&TV for a year with the support of the M&TV community. Also see the comments below for other examples.
The reality is, as long as you are not spamming sites that do not relate and you're not encouraging your users to abuse voting privileges by upvoting the ad on a site they don't use, you should be fine.
What you likely can't see on that topic is the Boating.SE ad that was deleted... because boats have nothing to do with Movies or TV... Consider your audience.
I have noted this in the past on an answer to a related question here. Which was also discussed on the linked Area 51 discussion here. Please note the question's edit history and the long list of unrelated sites that this proposal got spammed to... no, neither Ubuntu nor Bicycles have any connection to Arts and Crafts.
Now, note that the default ads will already appear on most sites so if you really want the target community to take your ad seriously, I recommend you do so, too... create one from scratch rather than using the default one.
Have fun with it. Make the imagery and text apply to the site you're targeting... Take a peek at the other ads in the M&TV Community Ads topic (or any site's topic) as examples.
Finally, be prepared to have your ad downvoted - particularly if you don't consider the above advice - and be gracious about it. If you get downvotes, ask why rather than getting defensive. Perhaps the site doesn't see the connection that you do... think of it as a process, not a one-shot deal.