I have the same board and I have this case and I love it!
I purchased this portable hardcase from Amazon (currently $33.77) for my DDJ-400, and I love it: https://www.amazon.com/dp/B01JGQVQBU?ref=ppx_pop_mob_ap_share
Still haven't ordered it yet. TBH i'm a little annoyed with gimbals that dont disassemble so i'm considering just getting a Moza Aircross 3 or RSC2 or something that folds down into an actual rectangle that can be stored in my existing cases.
Still, for $20-30 i'm considering just picking one up and seeing how i like it. Cases can always be repurposed for other gear in the future and the size of these cases is "Weird" enough to where it might have a cool use case for other gear or electronics.
https://www.amazon.com/gp/product/B09JFW4WZ9/ref=ox_sc_act_title_2?smid=A2AFHGCIAC5WKS&psc=1
https://www.amazon.com/gp/product/B01JGQVQBU/ref=ox_sc_act_title_3?smid=A3JF04FFC06LR0&psc=1
Also look up "diy trekpak". For like $20 you can make your own custom case dividers that work way better than foam or the velcro soft ones. I'm considering making up a batch and velcroing it to the sides to make it a more rigid case and have compartments for accessories if i do go that route. I use it in my main Pelican and it's worlds better than the alternatives and like 1/5 the price of the Trekpak brand for the same thing.
Like this? https://www.amazon.com/gp/aw/d/B01JGQVQBU
I have this $35 one from amazon. Definitely nothing special: https://www.amazon.com/gp/product/B01JGQVQBU/
I also have this backpack that it fits in: https://www.inateck.com/products/40l-17-inch-carry-on-travel-backpack-bp03001-black
and I use a decksaver when it's in that.
• Hard Case for traveling with it.
• RCA to XLR cables because the DDJ-400 only has RCA outputs and he wouldnt be able to plug directly into speakers with RCA.
• Good headphones that aren't Beats By Dre
• Small Portable SSD hard drive with ample space for music. Avoid HDD as they are disk based and you can lose all your data if dropped.
I bought this one. Much cheaper and seems to work fine for my purposes.