The "slate-colored" is actually just a subspecies of the Dark-eyed Junco. So you actually have a Dark-eyed Junco (Junco hyemalis) no matter which subspecies it is. The subspecies exist as populations in certain regions of the country. Those listed in my field guide are: 1.) Pink-sided 2.) Oregon 3.) Gray-headed 4.) Slate-colored Each subspecies looks a little different. As for the slate-colored (the subspecies I see in my location), the first-winter males look very similar to the adult females (brown instead of slate gray). They can be difficult to age and sex unless it's the adult male, and the adult male is dark slate gray with a white belly and white outer tail feathers. Very distinct plumage on the slate-colored Dark-eyed Junco.

i live in south west colorado near Durango. My back yard is pinyon and juniper trees and oak brush with native grasses and some shrubs. I don't put out suet trays or raw suet till the winter storms start. the more it snows and blows the more winter birds i get. Mostly juncos with some chickadees and nuthatches. a few towhees and sparrows in winter along with woodpeckers and a few finches. two familys of jays year around with some magpies and starlings and summer oeioles and red wing blackbirds. I started with the metal cages nailed to sides of trees but the towhees and juncos tried but had little success eating from them but the hanging types such as woodpecker and chickadees and nuthatches liked them so i hung a block of wood with a large deep lid on the block to make a platform feeder so the towhees and juncos and the ground feeders could perch either on the sides of the lid or in it to eat. i make up my own suet mixes according to what i have on hand. usually lard or shortning or leftover cooking oils from the deep fryer or from cooking meats ect. i soften it in the microwave just enough to mix in the meat scraps and oats with nuts or dried fruits and berrys and thicken it with cornmeal or grits and laddle it into the plastic trays that raspberrys or blueberrys or other berrys come in from the grocery stores. put the plastic trays in the cages and hang the cages from the limbs. usually close to another limb that the birds can perch on to eat from the cages. If the starlings try to take over the suet trays i hang them away from other limbs so the birds have to hang onto the cages to eat. I scare off the starlings everytime i see them by tapping on the window

About four or five years ago, when I began actively feeding birds suet, I had a lot of Juncos. I try to add scraps to a tray that sits on my central air unit, and that's where they'd typically eat, but they'd also venture to my hanging suet. I also read that they love fritos, so perhaps adding crushed fritos to my suet attracted them as well. I don't use black sunflower seeds because the hulls contain a toxin that inhibits plant growth - and I'm an avid gardener / landscaper. In addition to suet I also provide nyger seed, safflower seed and wild mix bird seed. Ever since I added millet to my suet the wrens stuck around and refuse to leave. Huge mistake. At the same time I've added more evergreen trees & shrubs to my landscape, and they love the shelter it provides near my feeders. Especially considering we have chicken hawks (coopers hawks) in the area, so I try to keep my feeders near brush or trees.

I think the bracing on either side of the rail would hold it, even with a rounded top to the rail. You could always ask the maker since Etsy gives a way to contact them. You could also try somewhere like a big box store to see what they have for platform feeders or even somewhere like a local nature type store. You also might be able to jury rig something using bungee cords or a C clamp or even a small piece of plywood and 2x4s and a couple of nails. You might not need this long term and I hate to see you spend a lot of money on this. It might just be a matter of having the feeder available earlier in the season so that they continue coming in cold weather. It may just me a matter of patience.